Raw sea urchin gonads immediately after taking out from tests, that is, edible meats of sea urchin have a delicate taste, wherein the smell of the sea is mixed with the characteristic sweetness, and thus are widely favored. When allowed to stand, however, the sea urchin gonads suffer from melting followed by meat-breakdown within a half day. Attempts have been made to prevent the meat-breakdown, namely, to establish an effective method for the preservation of raw sea urchin gonads. Known chemical processes comprise treating raw sea urchin gonads with common salt at a concentration closely similar to seawater or adding alum to raw sea urchin gonads while controlling the concentration of common salt so as to constringe the raw sea urchin gonads. On the other hand, known physical processes involve a partial-freezing method wherein raw sea urchin gonads are maintained in a partially freezing state. Furthermore, it has been attempted to combine these chemical processes with the physical ones.
The consumption of raw sea urchin gonads in Japan has been more and more increasing. Recently, a large amount of raw sea urchin gonads produced in foreign countries are imported to Japan so as to satisfy the demand. Thus the output of imported raw sea urchin gonads overwhelmingly exceeds that of domestic ones at present in Japan. Since imported raw sea urchin gonads are processed at the production place before transporting, it is an important problem how to maintain a freshness of raw sea urchin gonads for a long time. From the viewpoint of taste, the inherent flavor and sweetness of raw sea urchin gonads would be deteriorated with the lapse of time. With the recent tendency toward gourmandism, consumers favoring the inherent taste of raw sea urchin gonads are increasing. Thus it is important to establish an improved process whereby not only the shape of raw sea urchin gonads but also the inherent taste thereof can be maintained.
Each of the known preservation processes as described above has some disadvantages and thus is unsatisfactory. In the case of the chemical processes, the treatment with common salt alone can maintain the shape of raw sea urchin gonads immediately after the treatment for a day at the longest. The subsequent meat-breakdown would significantly deteriorate the commercial value of the raw sea urchin gonads. When common salt is used together with alum, the shape of the raw sea urchin gonads immediately after the treatment can be maintained for only 2 or 3 days. The subsequent meat-breakdown would significantly deteriorate the commercial value of the raw sea urchin gonads. Thus it is said that the effective period of this treatment is limited to 4 or 5 days at the longest. In addition, alum shows a rough, bitter or astringent taste per se. Therefore, the use of a large amount of alum affects the taste of the raw sea urchin gonads. The inherent flavor and sweetness are lowered with the lapse of time, which makes the astringent or bitter taste of alum further conspicuous. Thus the taste of the raw sea urchin gonads is significantly deteriorated.
On the other hand, the partial-freezing method, which is a typical physical process, is advantageous in that a freshness of raw sea urchin gonads can be maintained in a partial-freezing state (-1.degree. to 3 .degree. C.) for a long time. However, an increase in temperature would cause meat-breakdown at an increasing rate. Furthermore, this process requires an elevated distribution cost, which is a disadvantage from an economical viewpoint.